This invention pertains to vehicular transport and, more particularly, to methods and apparatus for the sensing the position of, and communicating with, vehicles on a guideway. The invention has application, for example, in track- or guideway-operated vehicular systems, e.g., in on-board tracking of vehicle position and relative distance and in communicating between vehicles and between a vehicle and the wayside.
In the prior art, many different position-sensing systems have been proposed and placed into commercial use. The requirements for modern, automated transportation systems place stringent demands upon such systems for accurate, reliable information. In the case of Personal Rapid Transit (PRT), for instance, headways of one second or less have been proposed. With such short headways, it is imperative to have accurate, up to date position information to prevent vehicle collisions and operate in a safe manner.
A leaky waveguide implementation of a relative position-sensing mechanism has been proposed for use in transportation. This technique is very similar to RADAR within a waveguide, where a radio frequency signal is introduced in the waveguide and distance to the preceding vehicle is measured by the time delay of the echo. The waveguide is designed in such a manner that the radio frequency signals are constrained to travel, for the most part, within the waveguide. Since the waveguide is open on one side to allow these signals to be introduced, the signal strength decays with distance as some of the signal `leaks` out of this opening. Such a system tends to be fairly expensive to implement, in terms of both the waveguide and the radio frequency transmitter and receiver.
In order to acquire the range necessary for transit systems, a fairly expensive waveguide is typically necessary in such a system. Range is fairly limited, and is affected by weather conditions, limiting the usefulness of such a technique since the system must be designed for worst case scenarios. Although distance is fairly accurately measured, this technique has been implemented only to measure the distance to the preceding vehicle.
Another system proposed by the prior art for relative position-sensing is the discrete circuit transmission line. The transmission line consists of three parallel conductor cables bridged by discrete resistors at regular intervals in a ladder-type network. In this technique, each vehicle injects a sinusoidal signal into the transmission line through an antenna. The signal decays exponentially in the transmission line according to distance from the source. A following vehicle can detect the signal and extract approximate distance from the signal magnitude.
This system suffers to a small degree from component drift, and to a larger degree from variations in gap distance and track irregularities. With a small change in gap between antenna and transmission line, there may be a large change in signal magnitude. Thus, if the gap is slightly larger than nominal, a following vehicle would detect a lower level signal and determine that it is farther away from the preceding vehicle than the actual distance. The accuracy of the system also degrades with increasing distances between vehicles, since the signal decays exponentially. One particular implementation of the transmission line was fairly expensive to build due to its discrete nature and the inclusion of ferrite slugs in the guideway at regular intervals to improve performance. Again, this system only allows distance to either preceding or following vehicles to be detected. A bi-directional system could be implemented, however, by utilizing two sets of transmitters at different frequencies.
A further prior art system capable of relative position-sensing is that of low cost vehicle RADAR. Such a system utilizes a low power radar transceiver to detect the position of other nearby vehicles or radar reflectors on those vehicles. In order to hold costs in check in some proposals, no high power RF amplifiers are used and thus only low power signals are utilized; range is very limited in this system. This system is also limited to line of sight measurements, and thus range is very limited on guideway curves. This system had been proposed for use in warning systems on automobiles as well as Group Rapid Transit (GRT) systems. Since this type of radar system is not yet in mass production, the cost is still fairly high.
Many systems also exist in the prior art to allow a vehicle to detect its own position on the guideway. One widely used conventional system used for this purpose is that of an odometer. Odometers suffer from the accumulation of error over fairly long distances. Odometers also depend upon wheel traction for an accurate measurement of position. In icy conditions or other conditions in which wheel traction is lost, odometers may not operate properly.
The prior art also includes the use of markers along a guideway for the detection of position on board a vehicle. Such markers are typically spaced at regular intervals along a guideway, and thus only update position periodically. Such markers may be of optical, magnetic, inductive, or ultrasonic varieties.
Another technique proposed in the prior art is the utilization of the Global Positioning System (GPS) to acquire position information on board a vehicle. A direct implementation is not adequately accurate for many transportation systems. Differential positioning using the GPS has been proposed to circumvent this limitation. Other limitations include inadequate signal strength in underground installations and multipath interference in urban areas.
Inductive loops and ladder transmission lines (e.g., of the types disclosed in U.S. Pat. Nos. 3,772,640, 3,906,436, and 3,979,091) have also been utilized in the prior art to determine, at a wayside location, the approximate position of a vehicle on a guideway. The resolution of such systems are in discrete steps, depending upon the transposition periods utilized in the transmission lines. Related techniques have been used to instruct a vehicle of its proper location on a guideway, by placing a signal in a transposed winding which is inverted every time a sensor is supposed to cross the transposition. Thus, during proper operation, the sensor does not see the inversions of the signal.
A variety of methods have been used in the prior art to enable communication between a vehicle and the wayside. Most of these methods involve the use of wireless radio communication, wherein the spectrum used is regulated by government agencies. One disadvantage of such schemes is signal degradation from intervening structures such as buildings and tunnel walls. Another problem with such schemes is far field interference from other radio frequency sources.
Some inductive communication schemes have been proposed (e.g., in U.S. Pat. Nos. 3,979,091 and 3,617,890) to alleviate these problems. These schemes use inductive loops with regular transpositions to transmit and receive signals, reducing the effects of far field sources and far field emissions.
In view of the foregoing, an object of the invention is to provide improved methods and apparatus for guideway-based position sensing and communication.
Another object of the invention is to provide such methods and apparatus as can be applied to sensing the positions of, and communicating with, vehicles operated on or in conjunction with a guideway.
Yet another object of the invention is to provide such methods and apparatus as can be applied to all types of vehicles, regardless of whether they travel over rail, road or other mediums.
A still further object of the invention is to provide such methods and apparatus as permit on-board detection of vehicle position, as well as wayside-based detection.
Yet still another object of the invention is to provide such methods and apparatus as permit vehicle detection and communication, e.g., regardless of whether the vehicle is underground or otherwise obscured from contact with a ground station antenna or satellite.
Still yet another aspect of the invention is to provide such methods and apparatus as provide highly accurate position sensing.
These and other objects of the invention are evident in the drawings and in the disclosure that follows.